Maybe I misunderstand incorrectly what a "fire stop" is here. I thought it was stuff to plug the holes so no air could move through the hole in the 2x. With the hole filled, the center of the wire is still only 3/4" from free air on either side of the 2x.
It seems silly to derate the wire, since that means more holes in the fire stop.
In the NEC, derating of bundled conductors is required, with some exceptions. With fire stopping in hollow spaces, some of those exceptions disappear and you must derate.
The requirements on conductors penetrating walls, studs, floors, etc. with fire stopping all but dictate that the conductors pass through metal conduit.
If you think a bundle of conductors in free air will dissipate heat as well as the same bundle encased in fiberglass or foam, then it is understandable you find the explanation wanting.
Chuck
Which is what I'm saying is silly. If you plug a hole that a bundle of wires through you must derate, but if you don't plug it you don't? You *must* drill *more holes in the fire block?
*Damned* silly.
What does "all but" mean?
Through a 2x? No, I don't find the explanation wanting, I find is damned stupid! There has to be something more to the rule than you're saying (or know?).
Keith, I think what's confusing you is that the NEC simply doesn't address derating conductors passing through a hole in a 2x4. Anywhere. Period.
The NEC does talk about derating bundled conductors. Multiple conductors in a hole in a stud is a bundle and therefore would have to be derated.
But an exception in the NEC says a bundle doesn't have to be derated if it is less than 24 inches in length.
Another exception applies to nipples less than 24 inches in length (think conduit).
But if the bundle goes through fire stopping, the exceptions don't apply and you do have to derate.
Again, the 2x4 itself never enters into any discussion of derating a bundle of conductors in the NEC. It only addresses bundling, derating, and fire stopping.
Does that help?
Chuck
current-carrying
It isn't a shorter length in wood. The rules are the same for "bundled" cables and nipples - derating is not required if the length is under
24". Derating is for over 24" of "bundled" cables - not specifically wood. (310.15)If the hole in the stud is less 24"long, and the cables are not kept tight together after exiting the stud, derating is not required.
In the 2008 NFPA made it very clear they do mean that NM cables penetrating a framing member with any kind of fire or draft stopping must be derated. They added "caulk" to the list and specifically said you CAN'T use the 310.15 (A)(2) exception (that acknowlages thermal transfer down a wire). They also rejected Jim Pauley's proposal that you didn't have to derate if the fire/draft stopping was only on one side of the member (not actually in the hole). They are serious about this, even though it does sound silly. (not unusual for NFPA) Effectively this simply limits you to 9 current carrying conductors per hole so it is not really that big a deal.
On Tue, 1 Jan 2008 13:55:10 -0500 krw wrote: | In article , | alt.engineering.electrical, snipped-for-privacy@ipal.net says... |> On Mon, 31 Dec 2007 21:53:06 -0500 krw wrote: |> | In article | snipped-for-privacy@d4g2000prg.googlegroups.com>, |> | alt.engineering.electrical, snipped-for-privacy@electrician2.com says... |> |> On Dec 26, 7:15?am, "JC" wrote: |> |> > I've searched all through my 2005 NEC looking for some guidance as to how |> |> > many 12/2 circuits I can run through a single 3/4" hole in the wall studs of |> |> > a house I'm building as a retirement project. I can not find anything about |> |> > it. I've also asked on several over groups. Is there some rule of thumb or |> |> > is there an actual regulation that I'm missing. I'm way out in the country |> |> > and we do not have an inspector that I can ask and the one in the county |> |> > seat said there is no restriction. I'm not concerned about county |> |> > restrictions, I'm concerned about safety. |> |> >
|> |> > TIA |> |> >
|> |> > -- |> |> > JC from Gnat Flats |> |> |> |> There are no rules on this in the 2005 NEC. As long as you keep the |> |> cables separated and not bundled and do not apply fire stopping you |> |> can put as many cables through a hole in a stud and will fit. |> |> If you apply fire stopping you must derate according to the 2008 NEC. |> | |> | Doesn't that seem sorta silly? |> |> No. A simple short (2 inches) hole does not tend to impact the ability |> of the wires to carry heat away and dissipate it into air a few inches |> away. OTOH, a fire stop would be more constraining on the wires and |> could reduce the heat dissipation. | | Maybe I misunderstand incorrectly what a "fire stop" is here. I | thought it was stuff to plug the holes so no air could move through | the hole in the 2x. With the hole filled, the center of the wire | is still only 3/4" from free air on either side of the 2x.
Without the fire stop, the ability to dissipate is my multiple means. It can dissipate _some_ within the hole, and _some_ through the wires to the free air away from the hole. The fire stop would have the effect of reducing or eliminating _one_ of these means of heat escape. That is, afterall, its design purpose.
| It seems silly to derate the wire, since that means more holes in | the fire stop.
More holes would be if you reduce the bundling rate to _avoid_ derating or as much derating. Otherwise it means using _larger_ wire that is treated as rated less. The latter might have to be the way to do it if the number of holes is an issue (as building construction code or safe engineering may dictate).
I think the derating could be more flexible, such as a variant rating factor for the length of the run, and the heat transfer capability of the containing material. But this can also get complex and would need to be one of those "under engineering supervision" things (where a PE puts his license on the line).
A separate issue from 'ordinary' holes. It didn't register that this applied to simple seals that weren't fire stops. I wonder if the heat transmission of caulk is lower than fiberglass. What is the heat buildup of 20" of bundled Romex in fiberglass. Foam-in-place insulation should be as bad as firestop.
You start getting into the difference between Draft stopping, Fire Blocking and Fire Stopping The short answer; Foam is draft stop, just stops the free flow of air. Wood is fire block, slows the spread of fire products like smoke and fumes and retards the spread of the fire. Fire Stop is a U/L listed assembly using methods like red caulk and pads that is engineered for a particular burn through time. Simply squirting some red caulk in a hole is not necessarily a fire stop. It depends on the application.
Personally I am not a huge fan of the spray in foam. Some of it burns like those 4th of July "snakes" you scorched the sidewalk with.
Unless that 2x4 is a fire block. Silly.
See the rest of this discussion. Apparently there is an exception for runs under 24" that doesn't include fire blocks. At some point this cases *more* holes in the fire block. Damned silly.
Unless it's a fire block, then it must be derated.
Well, a nipple less than 24" *is* less than 24", so I guess that works.
So if you come up against that situation you have to drill another hole. That is the silliest thing I've heard in a long time.
It *certainly* does! Fireblocks *are* made out of 2x4s.
Helps clarify that I'm *NOT* missing anything and that the NEC is damned silly! ...and I thought most things in there made sense, if you looked at it from *some* angle.
It only limits the radial dissipation for 3/4" of the wire, I.e. not much.
If you have a constant load... Well, you finish the sentence.
Nonsense.
Didn't know there were that many kinds of stops/blocks.
I wasn't clear in my intent, which was to compare 20" of Romex bundling in wall cavities that are insulated with either fiberglass batts or spray-in-place foam. Would seem like foam in wall cavities would be worse than fire stopping in framing because there is more length to generate heat for the same heat transfer down the wire. If caulk/fire stop in framing with over 2 cables is a problem it should also be a problem with foam in wall cavities, and only 2 cables should go through framing holes, and cables should be substantially spread between holes.
I thought someone in this thread went back to the source for derating for over 2 Romexes in firestop. It was based on "experimentation" that found "temperatures well in excess of their 90 C ratings". Results apparently not published, so the applicability to draft stopping not in the hole, as described by gfretwell, is not know by mere mortals like us. Also not known if the experimentation was reasonable. The requirement could make sense from some angle and is still be silly.
As gfretwell pointed out, the effective limitation is "9 current carrying conductors per hole" since you derate from the 90 C ampacity, but you can only use Romex at a 60 C ampacity.
Installers should not be bundling cables in walls under any circumstances. They should be separated and stapled along the studs or hanging on "stackers". I do find one interesting paradox. You don't have to derate an 18" nipple in 312.5(C) ex that is packed with duct seal. Why is duct seal at the end of a nipple less of a problem than caulk in a framing member hole.
But nobody has ever accused NFPA of making sense
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